The present invention relates to a method for controlling multi-slot calls in order to efficiently utilize exchanges and their associated transmission equipment which are the major facilities for implementing multi-slot call communication of audio and data signals at different transmission rates. The invention also relates to a multi-slot call switching system and its associated transmission equipment operating on this method.
As discussed in a journal of the Institute of Electronics, Information and Communication Engineers (B-I, Vol. J72-B-I, No. 4, pp. 255-263, April, 1988), there are four representative prior art methods for improving the availability of telephone circuits for calls:
(1) Trunk reservation method
The trunk reservation method involves leaving a certain number of telephone circuits unoccupied in preparation for the incoming of priority calls. The remaining circuits are shared by priority and non-priority calls.
(2) Split high-usage group method
The split high-usage group method involves virtually splitting the telephone circuits into two groups, one shared by both priority and non-priority calls and the other dedicated to the priority calls only. The dedicated circuit group is placed before the shared circuit group to implement control over the loss probability by call type. The loss of priority calls ultimately occurs in the shared circuit group.
(3) Split final group method
The split final group method also involves virtually splitting the circuits into two groups, one shared by both priority and non-priority calls and the other dedicated to the priority calls only. What differs from the split high-usage group method is that the dedicated circuit group is placed after the shared circuit group to implement control over the loss probability by call type. Priority calls are allowed to select circuits first in the shared circuit group and then in the dedicated circuit group.
(4) Virtual circuit method
According to the virtual circuit method, the maximum number of concurrently connectable non-priority calls is set for N-v; if the number of concurrently connected non-priority calls is N-v upon arrival of a non-priority call, that non-priority call is treated as a loss.
By any of the methods outlined above, once circuits are reserved for calls, the circuits stay reserved until the calls terminate. Even upon arrival of calls that require less than the capacity of available circuits, the calls may not be connected because of the chronological call order that must be maintained. This often results in inefficient uses of telephone circuits.
The above prior art is generally practiced as follows. Where 2.sup.N (N: natural number) basic switching units are handled in a channel memory constituting a multi-slot call switching system, the calls are written to or read from the memory or they occupy transmission circuits in keeping with certain rules upholding the preservation of the chronological call order. One disadvantage of the prior art is that these rules may prevent the switching of 2.sup.N basic switching units even if the capacity of the channel memory or of the available transmission circuits is equal to or more than the 2.sup.N basic switching units.